1. Field of the Invention
The present invention relates generally to an apparatus and method for estimating certain mechanical properties of a ferromagnetic sample, such as steel, and, more particularly, relates to an apparatus and method for estimating the hardness of a sample of ferromagnetic low-carbon sheet steel by measuring and analyzing the Barkhausen signals measured from the sample when subjected to a varying external magnetic field. Preferably, the measurement can be made either on-line or off-line.
2. Background and Objects of the Invention
Approximately 65 million tons of sheet steel per year are produced in the United States. One of the challenges to the steel making industry today is to improve the ability to produce large quantities of sheet steel having well-controlled and uniform mechanical properties with a minimum amount of scrap steel produced. It is estimated that if the rejection rate of sheet steel could be cut in half, a savings on scrap on the order of $100 million per year could be realized. Still further benefits would result from the increased production efficiency realized by the end users of the steel and by reductions in the amount of the time-consuming and costly testing of the mechanical characteristics of the steel to be shipped to the end users, which is presently required.
Various mechanical properties of steel, such as hardness, strength, ductility and strain-hardening exponent, are structure-sensitive, and therefore are directly related to the microstructure and composition of the steel. In other words, the microstructure and composition of the steel directly affects these mechanical properties. As a result, if the microstructure can be analyzed, then the mechanical properties of the steel can be predicted.
The microstructure, composition, and processing of the steel also control, to a large extent, the magnetic characteristics of ferritic or ferromagnetic steel. For example, magnetic properties of these steels that are sensitive to microstructure include initial permeability, maximum permeability, coercive force, remanance, and the Barkhausen effect. Thus, if any of these magnetic characteristics of the steel can be measured reliably, then the microstructure of the steel can be predicted. In turn, the microstructure of the steel can then be used to predict the mechanical properties. Further, if sensitive measurements of the magnetic characteristics of the steel can be performed rapidly and on-line, then the mechanical properties can be predicted and the steel production process altered, as necessary, to customize the resultant characteristics of the steel.
Of the magnetic properties of steel mentioned above, the one which appears to the inventors to be most useful for predicting the hardness of the steel, either when the steel is stationary or while it is moving during processing, is the Barkhausen effect. As discussed in detail below, the Barkhausen effect may generally be characterized as a series of abrupt changes or jumps in the magnetization of a ferrous material (characterized by Barkhausen signals) when a magnetizing field applied to the material is gradually altered. The Barkhausen measurements discussed herein can also be combined with other on-line measurements, such as, for example, grain size, to improve predictive capabilities.
Although the Barkhausen effect in and of itself is a known scientific phenomenon, its use in the apparatus and methods of the particular preferred embodiments disclosed herein is novel and unobvious. For example, U.S. Pat. No. 4,599,563 to Tiitto et al. appears to disclose the general concept of sensing the Barkhausen signals generated within a steel specimen in a plurality of directions to determine anisotropic properties of the steel. Anisotropic properties are said to include hardness. (See, e.g., column 2, lines 15-27 and 47-52; and column 3, lines 3-15). However, the manner in which the Barkhausen signals are used by the Tiitto et al. '563 system differs substantially from that of the present invention. The particular features of the present apparatus and method are neither disclosed nor suggested by Tiitto et al. '563.
U.S. Pat. No. 4,481,470 to Wallace discloses a method for determining the amount of hardness in strain-hardened articles of tungsten-nickel-iron alloy by measuring the magnetic flux emanating from the article. The use of the Barkhausen effect is not disclosed.
Kurita, "Measuring Technology in Continuous Annealing", Transactions ISIJ, vol. 26, 1986, pp. 3-22, recognizes that the hardness of steel is closely related to the grain size of the steel. The hardness of rolled steel is measured by magnetizing the steel and measuring the leakage of the resulting magnetic field. Kurita relies on the premise that the leakage varies with the grain size of the steel and may therefore be used to determine hardness. Kurita also discloses magnetizing a strip of steel and measuring the residual magnetism. The hardness of the steel is determined as a function of the residual magnetism. Use of the Barkhausen effect is not disclosed by Kurita.
Karjalainen et al., "Detection of Fabrication Stresses by the Barkhausen Noise Method", Effects of Fabrication Related Stresses, September, 1985, pp. 149-161, and Kaplan et al., "Nondestructive Evaluation of Ferromagnetic Materials by a `Magnetometer Like` Experimental Arrangement", Journal of Nondestructive Evaluation, Vol. 6, No. 2, 1987, pp. 73-79, and U.S. Pat. Nos. 3,427,872 to Leep, 4,689,558 to Ruuskanen et al., 4,977,373 to Tiitto and 4,881,030 to Stuecker et al., each disclose systems for measuring Barkhausen signals to determine residual stress or fatigue limit in ferromagnetic materials, such as steel. Measuring hardness as a function of the Barkhausen signals is not disclosed.
Lastly, U.S. Pat. No. 4,634,976 to Tiitto discloses a particular sensor configuration employing the Barkhausen effect for detecting stresses and structural defects in metal. The particular features of the present apparatus and method, however, are neither disclosed nor suggested by Tiitto '976.
Accordingly, it is an object of the present invention to provide a steel hardness measurement system and method of using same by which the Barkhausen signals in a specimen of steel, such as low carbon, rolled sheet steel, are measured as a function of one or more of a variety of parameters, to give an indication of the hardness of the steel.
It is a further object of the present invention to provide a system and method for measuring the hardness of the steel by measuring the Barkhausen signal emission rate as a function of the magnetic field intensity H.
It is a further object of the present invention to provide a system and method for measuring the hardness of the steel by measuring the Barkhausen signal emission rate as a function of the magnetic field intensity H.
It is a still further object of the present invention to provide a system and method capable of measuring the Barkhausen signal in a steel sample that is subjected to various stress samples.
It is yet a further object of the present invention to provide a system and method for measuring the Barkhausen signals in either a stationary or moving sample of steel.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.